In this page, we will see how monsters know the difference between an ally, neutral and enemy.

Classification

All entities even non-monsters have a int CBaseEntity::Classify() method that returns its classification that determines which kind/faction/team it is part of.

Here's a table of all existing classifications (cbase.h):

Constant	Value	What does it mean for the entity
CLASS_NONE	0	This entity does not need a classification.
CLASS_MACHINE	1	This entity is a machine (sentry, turret).
CLASS_PLAYER	2	This entity is a player.
CLASS_HUMAN_PASSIVE	3	This entity is a passive human (scientist).
CLASS_HUMAN_MILITARY	4	This entity is a human with military knowledge (Barney).
CLASS_ALIEN_MILITARY	5	This entity is an alien with military knowledge (alien grunt).
CLASS_ALIEN_PASSIVE	6	This entity is a passive alien (not used by any existing monster).
CLASS_ALIEN_MONSTER	7	This entity is a not so friendly alien monster (Big Momma, Gargantua).
CLASS_ALIEN_PREY	8	This entity is an alien prey for predators (headcrab).
CLASS_ALIEN_PREDATOR	9	This entity is an alien predator on the hunt for preys (bullsquid).
CLASS_INSECT	10	This entity is an insect (cockroach).
CLASS_PLAYER_ALLY	11	This entity is an ally for the player (Barney & scientist).
CLASS_PLAYER_BIOWEAPON	12	This entity is an alien projectile fired by a player (player hornets).
CLASS_ALIEN_BIOWEAPON	13	This entity is an alien projectile fired by an alien monster (alien grunt hornets).
CLASS_BARNACLE	99	Special classification for barnacles.

Relationships

Like in real life, a relationship dictates how an entity (mostly monsters) should behave towards another.

Whenever a relationship needs to be tested, a call to the method int CBaseMonster::IRelationship(CBaseEntity* pTarget) is made and returns one of the following values (monsters.h):

Constant	Value	What does it mean for the monster
R_AL	-2	Ally - This monster is my ally and a friend.
R_FR	-1	Fear - I am afraid of this monster, I need to get away from him.
R_NO	0	No relationship - Not my friend but not my enemy either.
R_DL	1	Dislike - This is an enemy I need to attack him.
R_HT	2	Hate - I hate this enemy and I will attack him in priority.
R_NM	3	Nemesis - I really hate this enemy and I want him dead no matter what.

A hard-coded matrix is used to determine the relationship between two entities based on their classification, that matrix is (static int iEnemy[14][14] defined in int CBaseMonster::IRelationship(CBaseEntity* pTarget)):

In the following image, prefixes (CLASS_ and R_) are omitted for the sake of clarity. The way to read this matrix is simple, the correct line is the entity's classification determining the relationship (this) and the column is the target's (pTarget) classification.

Keep in mind that entities can override int CBaseMonster::IRelationship(CBaseEntity* pTarget) and thus have a specific relationship with one or more specific entities in specific cases.

Let's take a look at the human grunt (int CHGrunt::IRelationship(CBaseEntity* pTarget)) relationship code for an example:

int CHGrunt::IRelationship(CBaseEntity* pTarget)
{
    if (FClassnameIs(pTarget->pev, "monster_alien_grunt") || FClassnameIs(pTarget->pev,  "monster_gargantua"))
        return R_NM;

    return CSquadMonster::IRelationship(pTarget);
}

We can determine that human grunts are really pissed off against alien grunts and gargantuas that they want them dead at all costs because they treat them as "nemesis".

There is no override of int CBaseMonster::IRelationship(CBaseEntity* pTarget) in CSquadMonster (the human grunt's base class) so we fall back to the matrix shown above defined in CBaseMonster instead.

From there, we have multiple cases depending on the target:

• They hate players, alien military and player allies like Barney (R_HT on CLASS_PLAYER, CLASS_ALIEN_MILITARY and CLASS_PLAYER_ALLY).
• They dislike passive humans like scientists, passive aliens, alien monsters like Vortigaunts, alien preys like headcrabs, alien predators like bullsquids (R_DL on CLASS_HUMAN_PASSIVE, CLASS_ALIEN_PASSIVE, CLASS_ALIEN_MONSTER, CLASS_ALIEN_PREY and CLASS_ALIEN_PREDATOR).
• They ignore the rest (R_NO on classifications not mentioned in this list).